Recent emergence of the high-temperature superconducting (HTS) thick-film tape technology is expected to meet the cost, size and performance requirements of superconducting components needed for advanced power applications for the defense and commercial sectors. One of the major potential HTS applications is in the electric power industry.
The YBa2Cu3O7 and related ceramic materials (YBCO) have appropriate intrinsic properties in the liquid nitrogen temperature range. However, their properties are drastically affected by grain boundary misorientations. Hence, in order to enable applications of HTS at high temperature and high fields, it is imperative that a biaxially textured, polycrystalline YBCO tape, or related article, be developed which contains a minimal number of high angle grain boundaries.
One of the industrially scalable processes for producing biaxially textured YBCO conductors is by using Rolling Assisted Biaxially Textured Substrates (RABiTS). As described more fully below, a biaxially textured, flexible metal/alloy substrate is formed by conventional thermomechanical processing followed by epitaxial deposition of buffer layer(s), YBCO grown on such substrate often exhibited Jc's over 1 MA/cm2 at 77K. To date, the preferred buffer layers for the RABiTS approach have been the combination of CeO2 and YSZ. However, these oxide buffer layers lack important properties, e.g., electrical and thermal conductivity and mechanical toughness. It has been a challenging engineering task to develop a large-scale continuous process for producing thick (>0.5 μm) crack- and pore-free oxide films. Microcracking in oxide films is commonly observed in thick films due to the brittle nature of the oxide materials. Microcracks in the oxide layer can serve as open paths for oxygen to diffuse and oxidize the underlying metal during subsequent YBCO processing. Finally, the oxide deposition step on the Ni substrates is difficult; high quality films are only obtained by using very low deposition rates. In addition, as with many HTS applications, conductive buffer layers are needed since they would provide electrical coupling of the HTS layer to the underlying metallic tape substrate. This is an important property in order to electrically stabilize the conductor during transient loss of superconductivity in some applications.
Numerous applications of high temperature superconductors, such as transformers, generators and motors require high current carrying, flexible conductors which can sustain magnetic fields above 0.1 T. Due to the thermally activated flux flow, the critical current density of most of the highly anisotropic superconducting compounds, such as the Bi-based compounds, rapidly drops at 77K in the presence of an externally applied magnetic field. Moreover, it is not clear if the uniaxially textured, Bi-based wires which typically contain numerous high angle grain boundaries and have approximately ⅔ of their cross-sectional area occupied by silver, will ever reach adequate cost/performance levels for large scale commercial applications. Hence, the development of a viable and low cost processing route based on (Y or Re)Ba2Cu3Ox (YBCO) is of great interest currently and forms a central research thrust in the area of high temperature superconductivity, YBCO compounds have favorable intrinsic properties. Epitaxial YBCO thin films on single-crystal substrates yield critical current densities (Jc's) in the range of 106-107 A/cm2 at 77K, 0 T. YBCO films also have a high irreversibility field of ˜6 T at 77K.
Conventional ceramic fabrication methods which can be used to make a long, flexible conductor result in materials with weak, if any, macroscopic or microscopic biaxial texture. In particular, YBCO materials fabricated using conventional techniques invariably contain numerous high angle grain boundaries. High angle grain boundaries act as Josephson coupled weak-links leading to a significant field-dependent suppression of the supercurrent across the boundary. For clean stoichiometric boundaries, the grain boundary critical current density depends primarily on the grain boundary misorientation. The dependence of Jc(gb) on misorientation angle was first determined by Dimos et al. in YBCO for grain boundary types which can be formed in epitaxial films on bicrystal substrates. These include [001] tilt, [100] tilt, and [100] twist boundaries. In each case high angle boundaries were found to be weak-linked. The low Jc observed in randomly oriented polycrystalline HTS fabricated using conventional methods can be understood on the basis that the population of low angle boundaries is small and that frequent high angle boundaries impede long-range current flow. Hence, controlling the grain boundary misorientation distribution towards low angles is key to fabricating high-Jc materials. Practically speaking, this limitation entails the fabrication of biaxially textured superconductors.
Successful fabrication of biaxially textured superconducting wire based on the coated conductor technology, requires optimization of the cost/performance of the HTS conductor. From a superconducting performance standpoint, a long, flexible, single crystal-like wire is required. From a cost and fabrication standpoint, an industrially scalable, low cost process is required. Both of these critical requirements are met by Rolling-assisted-biaxially-textured-substrates (RABiTS). However, in order for cost/performance for a conductor based on this technology to be optimized, further work needs to be done in the area of buffer layer technology. It is now clear that while it is fairly straightforward to fabricate long lengths of biaxially textured metals or alloys, it is quite difficult to deposit high quality buffer layers using low cost processes. Requirements of buffer layers include—it should provide an effective chemical barrier for diffusion of deleterious elements from the metal to the superconductor, provide a good structural transition to the superconductor, have a high degree of crystallinity, excellent epitaxy with the biaxially textured metal template, have good mechanical properties, high electrical and thermal conductivity and should be able to be deposited at high rates.
Buffer layers of the prior art include use of YSZ and CeO2, typically a configuration of CeO2 (0.01 μm)/YSZ (0.5 μm)/CeO2 (0.01 μm). The purpose of the first buffer layer is to provide a good epitaxial oxide layer on the reactive, biaxially textured Ni substrate without the formation of undesirable NiO. CeO2 is special in its ability to very readily form single orientation cube-on-cube epitaxy on cube textured Ni. Deposition of CeO2 using a range of deposition techniques is done using a background of forming gas (4% H2-96% Ar) in the presence of small amounts of water vapor. Under such conditions the formation of NiO is thermodynamically unfavorable while the formation of CeO2 is thermodynamically favorable. The water vapor provides the necessary oxygen to form stoichiometric CeO2. It is not possible to deposit YSZ under such conditions with no evidence of undesirable orientations. In the case of CeO2 one can readily obtain a single orientation, sharp cube texture. Ideally, it would be desired that the CeO2 layer be grown thick such that it also provides a chemical diffusion barrier from Ni, followed by deposition of YBCO. However, when the CeO2 layer is grown greater than 0.2 μm in thickness, it readily forms micro-cracks. Hence a YSZ which does provide an excellent chemical barrier to diffusion of Ni and does not crack when grown thick is deposited on a thin initial template of CeO2. Initial demonstrations at ORNL were using this configuration. However, since there is a significant lattice mismatch between YSZ and YBCO (˜5%), a second 45°-rotated orientation nucleates at times. In order to avoid the nucleation of this second orientation completely, a thin CeO2 layer is deposited epitaxially on the YSZ layer. This completes the buffer layer structure. YBCO can now be deposited on the layer which has an excellent lattice match with YBCO (˜0.1%).
The drawbacks of this buffer layer structure are that the deposition of the first CeO2 layer is non-trivial. Strict control of deposition conditions in particular, the O2 partial pressure is required to avoid formation of undesirable NiO (NiO typically nucleates in mixed orientations and is also very brittle). Furthermore, CeO2 can have wide range of oxygen stoichiometry. It is brittle and is not conducting. It will be a challenging engineering task to develop a large-scale continuous process for producing thick (>0.5 μm) crack- and porosity-free oxide films based on a vapor phase process. For example, in a continuous process involving reactive electron beam evaporation of Ce to form CeO2, issues relating to the formation of an oxide on the target complicate matters relating to rate of deposition as well as stability of the melt pool. Any change of conditions during deposition are known to have profound affects on the film microstructure. Moreover, any oxidation of the biaxially textured metal, even after the successful deposition of CeO2, can induce undesirable interfacial stresses leading to spallation or further cracking, thus deteriorating the material properties. Microcracks in the oxide buffer layer will adversely affect the epitaxial quality of the growing YBCO film and create weak-links, besides serving as diffusion paths for Ni. Lastly, the surface morphology of the buffer layer is important for subsequent YBCO growth. Ideally, it would be desired to have a buffer layer which tends to be smoother than the Ni substrate it is grown on. In other words, the buffer layer has a sharper rocking curve than the underlying substrate. For CeO2 the rocking curve is almost the same as that for the Ni substrate. All things considered, buffer layer deposition of the prior art is time-consuming and qualitatively deficient.
Macroscopically, biaxially textured YBCO conductors have been formed by epitaxial deposition of YBCO on flexible substrates formed by two techniques—(a) A flexible, unoriented, polycrystalline metal substrate coated with an oxide buffer film(s) with a forced biaxial texture induced by ion-beam-assisted-deposition (IBAD), where an assisting noble gas beam extracted from an ion source is directed onto the growing film. A similar biaxial texture is observed during oblique vapor deposition on an inclined polycrystalline substrate and (b) A biaxially textured, flexible metal based substrate formed by conventional thermomechanical processing followed by epitaxial deposition of buffer layer(s). This technique is referred to as Rolling Assisted Biaxially Textured Substrates (RABiTS). Using both these techniques Jc's over 1 MA/cm2 at 77K have been achieved.
The foregoing background information, together with other aspects of the prior art, are disclosed more fully and better understood in light of the following publications:    1. Kormann, G., Bilde, J. H., Sorensen, K., de Reus, R., Anderson R. H, Vace, P., and Fraltoft, T., “Relation between Critical Current Densities and epitaxy of Y-123 Thin Films on Mgo (100) and SrTiO3 (100),” J. Appl. Phys., 1992, 71, 3419-3426.    2. Matsuda, H., Soeta, A., Doi, T., Aikhara, and T. Kamo, “Magnetization and Anisotropy in Single Crystals of Ti-1223 of Tl—Sr—Ca—Cu—O System,” Jpn. J. Appl. Phys., 1992, 31, L1229-31.    3. Dimos, D., Chaudhari, P., Mannhart, X., and F. K. LeGoues, “Orientation Dependence of Grain Boundary Critical Currents in YBa2Cu3Ox Bicrystals,” Phys. Rev. Lett., 1988, 61, 219-222; Dimos, D., Chaudlhari, P., and Mannhart, J., “Superconducting Transport Properties of Grain-boundaries in YBa2Cu3Ox Bicrystals”, Phys. Rev. B, 1990, 41, 4038-4049.    4. Iijima, Y., Tanabe, N., Kohno, O., and Ikeno, Y., “In-plane Aligned YBa2Cu3Ox Thin-Films Deposited on Polycrystalline Metallic Substrates,” Appl. Phys. Lett., 1992, 60, 769-771.    5. Reades, R. P., Burdahl, P., Russo, R. E., and Garrison, S. M., “Laser Deposition of Biaxially Textured Yttria-stabilized Zirconia Buffer Layers on Polycrystalline Metallic Alloys for High Critical Current Y—Ba—Cu—O Thin-films,” Appl. Phys. Lett., 61, 2231-2233, 1992.    6. Wu, X. D., Foltyn, S. R., Arendt, P. N., Blumenthal, W. R., Campbell, I. H., Cotton, J. D., Coulter, J. Y., Hints, W. L., Matey, M. P., Safar, H. F., and Smith, J. L., “Properties of YBa2Cu3Ox Thick Films on Flexible Buffered Metallic Substrates,” App. Phys. Lett., 1995, 67, 2397.    7. Hasegawa, K., Fujino, K., Mukai, H., Konishi, M., Hayashi, K., Sato, K., Honjo, S., Satao, Y., Ishii, H., and Iwata, Y., “Biaxially Aligned YBCO Film Tapes Fabricated by All Pulsed Laser Deposition”, Appl. Sppercond., 4, 475-486, 1996.    8. Goyal, A., Norton, D. P., Christen, D. K., Specht, E. D., Paranthaman, M., Kroeger, D. M., D. P., Budai, J. D., He, Q., List, F. A., Feenstra, R., Kerchner, H. R., Lee, D. F., Hatfield, E., Martin, P. M., Mathis, J., and Park, C., “Epitaxial Superconductors on Rolling-Assisted-Biaxially-Textured-Substrates (RABiTS); “A Route Towards high Critical Current Density Wire,” Applied Supercond., 1996, 69, 403-427.; A. Goyal et al., U.S. Pat. No. 5,739,086 and U.S. Pat. No. 5,741,377.    9. Norton, D. P., Goyal, A., Budai, J. D., Christen, D. K., Kroeger, D. M., Specht, E. D., He, Q., Saffian, B, Paranthaman, M., Klabunde, C. E., Lee, D. F., Sales, B. C., and List, F. A., “Epitaxial YBa2Cu3Ox on Biaxially Textured Nickel (100): An Approach to Superconducting Tapes with High Critical current Density,” Science, 1996, 274, 755.    10. Goyal, A., Norton, D. P., Kroeger, D. M., Christen, D. K., Paranthaman, M., Specht, E. D., Budai, J. D., He, Q., Saffian, B., List, F. A.; Lett, D. F., Hatfield, E., Martin, P. M., Klabunde, C. E., Mathis, J., and Park, C., “Conductors With Controlled Grain Boundaries: An Approach To The Next Generation, High Temperature Superconducting Wire,” J. Mater. Res., 1997, 12. 2924-2940.    11. Goyal, A., Norton, D. P., Budai, J. D., Paranthaman, M., Specht, E. D., Kroeger, D. M., Christen, D. K., He, Q., Saffian, B., List, F. A., Leo, D. F., Martin, P. M., Klabunde, C. E., Hatfield, E., and Sikka, V. K., “High Critical Current Density Superconducting Tapes By Epitaxial Deposition of YBa2Cu3Ox Thick Films on Biaxially Textured Metals,” Appl. Phys. Lett., 1996, 69, 1795.    12. W. D. Sproul, “Physics and Chemistry of Protective Coatings,” Ed. by W. D. Sproul, J. E. Greene, and J. A. Thornton (AIP Conf. Proc. No. 149, 1986, New York), p. 157.